We're seeking clarification on how other users interpret the term "required ventilation air" in the exception to Section G3.1.2.9.1 of Appendix G (ASHRAE 90.1-2010) for laboratory spaces.
Section G3.1.2.9.1 states:
"G3.1.2.9.1 Baseline System Types 1 Through 8
System design supply air flow rates for the baseline building design shall be based on a supply-air-to-room-air temperature difference of 20°F or the minimum outdoor air flow rate, or the air flow rate required to comply with applicable codes or accreditation standards, whichever is greater. If return or relief fans are specified in the proposed design, the baseline building design shall also be modeled with fans serving the same functions and sized for the baseline system supply fan air quantity less the minimum outdoor air, or 90% of the supply fan air quantity, whichever is larger.
Exception: For systems serving laboratory spaces, use a supply-air-to-room-air temperature difference of 17°F or the required ventilation air or makeup air, whichever is greater."
Our interpretation is that "required ventilation air" refers to the amount of supply air in the Proposed design (i.e., the total supply airflow to the space). This would then be used to determine the baseline supply airflow, if it is greater than what results from the 17°F delta-T calculation.
Is this a correct understanding? Or should "required ventilation air" be interpreted more narrowly as just the minimum ventilation air required per applicable codes or standards (e.g., ASHRAE 62.1), regardless of the actual supply air quantity in the Proposed design?
We interpret the exception as allowing use of the full proposed design ventilation air to avoid penalizing laboratory spaces, which are typically driven by pressure cascade and safety requirements, not just thermal loads.
We would appreciate any insights, interpretations, or examples from reviewed LEED projects.
